1. Field of the Invention
The present invention relates to high-strength light-weight pottery articles having lower specific gravity (e.g. 1.3 or less) than the specific gravity (2.3 to 2.5) of general pottery. Further, the present invention also relates to a process for producing such lightened pottery articles.
2. Background Art
At present, ceramic, porcelain and pottery contribute to a wide variety of fields in human life, but their high specific gravity is mentioned as one disadvantage of their characteristics. Their specific gravity is generally 2.3 to 2.5, and in view of the advantages of these materials, this weight is a great problem, and lightening is strongly desired. Further, in the fields of mainly building materials, fine arts, ceramic art and wall paintings, it is expected that if pottery having a beautiful glazed face, is capable of being arbitrarily carved and colored and being excellent in weatherability and thermostability, can be lightened, then its usability is further raised.
Some measures have been taken so far to lighten pottery. Such measures include (A) a method of mixing a foaming agent to form bubbles by calcination, (B) a method of mixing a previously foamed foaming agent (e.g. perlite etc.) in a main body of pottery, and (C) a method of mixing combustible materials (e.g. powdery polyesters and wood wastes etc.) and gasifying the mixture at high temperature to generate voids where such materials were present. (A) and (B) can be applied to porous materials (kaolin, sericite etc.) by mixing a foaming agent in cement materials, but formation of uniform and equal pores is difficult in the case of producing pore voids by mixing a foaming agent in clay and slip. In the case of (C), gasification occurs as the furnace temperature is raised and ignition may occur depending on the case, to destroy a main body of pottery. Accordingly, articles such as dishes etc. having a thickness of 20 mm or less can be produced, but those having larger thickness cannot be produced. In addition, any articles thus produced are very low in strength.
Further, there is the problem that significant shrinkage occurs in the case of general pottery upon drying and calcination after forming, and this significant shrinkage causes a scatter of the sizes of articles after calcination.
To solve such problems, Japanese Laid-Open Patent Publication No. 203,555 of 1988 or Japanese Laid-Open Patent Publication No. 52,781 of 1997 disclose a method of producing light-weight pottery or a light-weight pottery plate wherein a commercial powdery ceramic hollow body (silas balloon, glass balloon etc.) is mixed into raw clay, molded by extrusion molding or formed on a potter's wheel, or press-molded, and the resulting molded article is calcinated. However, when a commercial ceramic hollow body is merely mixed into e.g. the porous silas balloon etc., the resulting article after calcination is poor in strength, and a water-absorbing article only is obtained due to formation of open cells. In the case of the glass balloon, the article after calcination has high strength, but it does not have water absorption properties so that its water content is not uniformly distributed during extrusion molding by a clay kneader, the water is separated with time, and further its molding is difficult while there is a scatter of the qualities of the articles after calcination. In press forming, even if a highly strong ceramic hollow body is used, pressure is applied by press unevenly so that its hollow structure cannot endure the pressure, to undergo partial destruction, and water is absorbed, and there is a scatter of its strength, specific gravity etc., thus making press molding unfit for large-scale production of a molded body (pottery plate) having a large plane. When glass balloon etc. are used, a paste for a glaze can be made uneven.
Originally, ceramic clay increases plasticity as aging proceeds with time after the starting material is admixed sufficiently with water, and for this reason, its perfection is raised in manufacture and as an article, but there is the problem that stable kneading of ceramic clay is difficult when mixed with a ceramic hollow body not having water absorption properties. Further, there are cases where commercial organic binders etc. are used for ceramic clay lacking in flexibility, but there are a wide variety of binders which should be selected depending on uses and product specifications through troublesome experiences and experiments, and further the binder itself is expensive and gasified upon calcination to form open cells in the article itself, to make it difficult to produce a calcinated body having exclusively closed cells necessary for producing a high-strength body while water absorptivity is also raised, thus limiting its applicability. Further, there is also proposed a method of mixing soda silicate (water glass) etc. as inorganic binders when raw clay is kneaded, but such a method suffers from the problem that uniform dispersion is hardly achieved and further the ceramic hollow body is destroyed due to excessive kneading, and thus it is difficult to supply uniform and stable articles in this method too.
Furthermore, when the article after calcination obtained using such processes is destroyed and its surface is magnified under a microscope, it is seen that the ceramic body is not exfoliated from binders (ceramic clay etc.) in its rupture face, but destroyed and split into two pieces in almost all cases, and therefore, it is necessary to further reinforce the ceramic hollow body itself in order to improve the strength of the article.